Heat transfer



Aug. 2 8, T928; 1',B81,92

- J.- E. BELL HEAT TRANSFER Filed Sept. 17,- 1923 5 Sheets-Sheet l FURNACE,

p 16 IN\I/EN'ILOR, g Wa s/sue BY ATTORNEY J. E. BELL HEAT TRANSFER Aug. 213, 192

Fild Sept. 17, 1923 3 Sheets-Sheet 2 IIIIIIIIIIIIIIII 'IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII (II/IlI/lI/IIIIII 11111111111111! llllllllllllll PNIL W H a I l INVENTOR whee BY a/MM ATTORNEY Aug. 28, 1928.

1,681,926 J. E. BELL HEAT TRANSFER Filed Sept. 17, 1923 5 Sheets-Sheet 3 INVENTOR ATTORNEY Fania Aug. 28,- i I 1,681,926

UNITED, STATES PATENT OFFICE.

JOHN E. BELL, OI BROOKLYN, NEW YORK,-.ASSIGNOR'TO FOSTER WHEELER CORPORA- TION, 01 NEW YORK, N. Y., A. CORPORATION 01 NEW YORK.

:sm'r 'rnmsrnn.

Application filed September 1?, 192a. sen a1 1T0. 863,107.

The general object of my present invenpans C with perforated bottoms, and a bottion is to provide an improved method of, tom tray or pan C having an imperforate and apparatus for absorbing heat from a bottom which serves as a receiving vessel for source of heat and imparting it to an object a suitable liquid heat transfer agent, as mer- 5 or material to be heated; 11 carrying out cury,'-which in regular operation is continumy invention, I employ a fluid heat transfer ously-supplied to the uppermost tray C, and 55 vehicle or agent which I continuously circuflows downward from each upper tray C in late in a circuit including a heat absorption the series, through the perforations in the portion and a heat dispensing or cooling porbottom wall of the trays in a multiplicity of tion; and my invention is especially characfine wire like pencils or streams. The flue terized bythe division of the heating agent -.B'contains a set of trays C and C which may 60 into a multiplicity of fine streams in the heat be identical in construction and arrangement absorbing and cooling portions .of the circuwith those in the flue A." Means are prolating system, whereby I secure an unusuvided for passing the heat transfer liquid 15 ally'high coefiiecient of heat transfer, and a I from the bot-tom tray or receiving vessel C corresponding reduction in the volume and in each of the flues A and B, to the topmost 65 weight of the apparatus required to transfer tray 0 in the other flue. The means .shown a given amount of heat. for this purpose comprise a pump DA for The various features of novelty which passing the liquid friom the tray C in the characterize my invention are pointed out flue A to the to tray C in the flue B, and a with particularity in the claims annexed to similar pump BB which passes the liquid 7 and forming a part of this specification; for from the bottom tray C in the fine B to the a better understanding of this invention howtop tray C in the flue A and thus maintains ever, its advantages and specific objects ata continuous circulation of the heat transfer tained with its use reference should be had liquid through the system. In uses of theto the accompanying drawin s and descripinvention in which a bottom tray 0 in either 75 tive matter in which I have illustrated and flue is at a level above that of the top tray described a preferred embodiment of my in- C in the other flue, no pump may be required veutiou, 4 in the conduit connection between such trays.

Of the drawings: In the operation of the apparatus shown Fig. 1 is a diagrammatic representation of in Fig. 1, the mercury fiowing downward in so one embodiment of my invention; finely divided streams through the spaces Fig. 2 is a somewhat diagrammatic reprebetween the trays in the flue A absorbs heat sentation of another embodiment of the infrom the gases passing upward through that vention; flue, and gives up this heat in its passage in Fig. 2 is a small scale side elevation of the finely divided streams through the spaces furnace shown in Fig. 2; between the trays in the flue B. The bulk Fig. 3 is a section on the line 33 of Fig. of apparatus constructed as shoWnin'Fig. 1, 2 and of Fig. 4; which is required for the transfer of a given 40 Fig. 4 is a section onthe line 4-4 of Fig. number of heat units from the heating gases 3; and to the air to be preheated may be made very Fig. 5 is a section on the line 5-5 of- Fig. 3. small as compared with that of apparatus In Fig. 1 1 have illustrated the use of my heretofore used for this purpose. This reinvention in transferring heat from a fluid, sults from the very high Value of the heat such as the heating gases from a. furnace, transfer coefficient. which is obtainable in flowing upward through a flue A, to a cooler this manner. The rate at which heat is abfluid, as air for combustion in thefu'rnace, sorbed from or imparted to a pencil or wire flowing upward through a flue B. In the like body of small diameter, such as is flue A'are a series of super-imposed trays or formed by p the mercury stream issuing through each small tray bottom perforation, is in approximately inverse proportion to the s uare root of the diameter of the pencil or cy inder. For example, a metal wire or rod one eighth of an inch in diameter and swept by a stream of hot gas will absorb heat approximately twice as rapidly as will a rod of the same metal and length and one half inch in diameter swept by the same stream of hot gas when the temperatures of the two rods are the same, although the larger rod has four times the heat absorption surface of the smaller rod, and weighs sixteen times asmuch. The use of mercury as a liquid transfer medium in open contact with the fluid, substances between which the heat transfer is effected, as illustrated in Fig. 1, is highly satisfactory from the standpoint of thermal efliciency and bulk of apparatus required, but ordinarily is attended with an evaporation loss of the liquid heat transfer agent which is objectionable from the practical standpoint for most purposes of use.

Loss of the fluid heat transfer agentby evaporation can be avoided. while retaining characteristic advantages of the invention by placing the agent in an enclosed circulation system, the heat absorption and dispensing portions of which are each formed by a grou of tubes or hollow Wires of suitably sma 1 diameter. The use of a closed circulating system permits of the employment of other fluid heat transfer agents than mercury, which is practically the only transfer agent available for use in such an open circulating system as 'is illustrated in Fig. .1. Mercury while an ideal transfer agent from the standpoint of thermal efficiency and bulk required, has a relatively low specific heat, so that the relatively large weight required for a given heat transfer effect and its relatively high cost per pound form practical objectionsto'its use.

Moreover, it objectionably attacks practically all metallic substances with which it comes in contact except iron, steel, and aluminum, and

indeed attacks aluminum to some extent. In

practice for such a use as that of preheating air with heat absorbed from-relatively low temperature stack gases, I advantageously employ a suitable liquid hydrocarbon of hydrocarbon mixture which will not crack or vaporize to any significent extent under the liquid heat transfer medium is continuously circulated in regular operation through a,

of the heat dispensing device FA to the inlet of the heat absorbing device F, and a conduit 6 through which the fluid transfer medium passes from the outlet of the device F to the inlet of .the device FA. Advantageously the circulating system comprises, an expansion chamber H with a vent to the atmosphere which may normally be closed by a. suitable back pressurevalve H.

The heat absorbing apparatus F in the par ticular form illustrated consists of a number of similar units or sections f, each of which comprises abox-like casing open at top and bottom to the sections of the stack flue AA above and below it. The casing portion of each section 7 is formed by cast metal front and rear wall plates I and IA respectively,

and wrought metal side plates J connected to the front and rear plates at the corners of edges of the side plates J which may be bolted to supporting beams or plates mechanically connecting the sections and closing the spaces between the adjacent sides of adjacent sections f.

Disposed in the casing of each section are amultiplicity of thin walled tubes K of small diameter. As shown each tube K is looped to provide four parallel limbs and is arranged wlth the four limbs horizontal and in the same vertical plane. The tubes K in each sectron f are arranged in two sets, one above the other. Theupper ends 'of the tubes in the upper set are suitably secured in openings in the back wall of a chamber I formed in the front wall plate I. The lower ends of the tubes in the-upper set and the upper ends of the tubes in' the lower set are similarly se- Inn cured in openings in the back wall of a chamber I and the lower ends of the tubes in the lower set are secured in openings inthe back wall of the chamber I formed in the front plate I. The front walls of the chambers I, I and I, are formed by removable cover plates-or heads. The chambers I of the vanous sectlons f of the device F are connected each by a separate branch pipe M to an inlet header'M, and the various chambers I are similarly connected by branch pipes Mito an outlet header N. The fluid transfer agent flowing downward through the upper set of tubesK in each section, passes into the lower set oftubes K- through the corresponding chamber P. The tubes K require means for supporting them at intervals along the len h of each tube limb. The means shown for t purpose comprise vertical tube sheets extending transverse to the tubes and suitably spaced between the front and rear wall plates I and IA. These tube sheets are formed in the construct-ion illustrated, 'byyertical bari I side walls like sections P having tube recelving notches P in their side'edges. The various bars P forming each sectional tube sheet are secured together in place between the corresponding J by through bolts Q. Advantageously, as shown, adjacent tubes K in each set are vertically displaced with respect to one another so that the adjacent horizontal limbs of adjacent tubes are vertically displaced or staggered with respect to one another. This displacement is provided for as shown in Figs. 3, 4 and 5, by locatin the notches P at one side of each bar P at evels alternating with those at which the notches at the other side of the bar are located.

The device FA in the furnace air intake flue BA may be identical in construction with the device F. The inlet header M of the device F is connected to the outlet of a circulating pump, E. The pump'inlet is connected to an expansion tank H to which the outlet header N of the device FA is connected while the outlet header N of the device F is connected by the conduit e to the inlet header M of the device FA. Inthe assembled apparatus, the casing of each section 7 of the devices F and FA is thus traversed by a mass of closely spaced horizontal tube portions of small diameter. The smaller the diameter of these tubes, the higher the heat transfer coeflicient of the apparatus and the smaller the bulk of the apparatus required for producing a given heat transfer effect.

In practice however, the extent to which thediameters of the tubes K may be reduced with advantage is limited, in part by the fact that the cost of manufacture and assembly of apparatus of given capacity increases as the size of the tubes diminishes, andby the fact .that very small tubes lack strength to resist deformation; but the main limitation on they extent to which the tubes can be reduced in diameter with advantage-is the resistance to the flow of the liquid heat transfer agent through the tubes, which, when the latter are made unduly small, results in an'undesirably high circulating pump delivery pressure and loa The last mentioned limitation isespecially important because a full realization-of the advantages of the counter current flow effect.

and the \maximum heat transfer'efi'ect possible in a given installation of the character illustrated in Figs. 2,-3, 4, and 5, requiresa particular and relatively high velocity of flow of the fluid heat transfer mediumithrough the tubes K. This velocity for maximum efli- In the foregoing equation, A is the product of the weight of the heat transfer agent flowing per unit of time into either of the devices F and FA, multiplied by the specific heat ofsaid agent; B is the product of the heat transfer rate in the device F multiplied by the heat absorbing surface area of that device; C is the product of the heat transfer rate in the device FA multi lied by the heat dispensing surface area of that device; D is the product of the weight of the heating gases flowing into contact'with the device F per unit of time multiplied by the specific heat of the heating gases and E is the product of the weight of air flowing into contact with the device FA multiplied by the specific heat of the air.

It will be apparent from the foregoing equation that in such a use of the invention in an air heater as is illustrated in Figs. 2, 3, 4, and 5, wherein the weight of the air to be preheated is only five per cent or so less than the weight of the heating gases and the specific heats of the air and heating gases are approximately equal, the weight of the fluid heat transfer agent flowing through each device F and FA per unit of time, will be in approximately in the same proportion to the weight of the heating gases flowing ast the device F in the same time, as exists etween the liquid transfer agent.

By way of'illustration and example, rather than by way of limitation,'I may state that the particular apparatus illustrated in Figs. 2, 3, 4, and 5, was designed bore is one eighth of an inch, and that the heat transfer apparatus comprising five units in each ofthe absorbing devices F and FA, with one hundred ninet two tubes, each about fourteen feet long, 111 each unit or section, was devised and is adapted to heat the necessary air for combustion of a coal fired boiler furnace from-an initial temperatureof F.,. to a final temperature'of 360 F., when theboiler is developing 2,000 boiler horse power per hour. With the proportions s ecified, and assuming a normal condition- 0 combustion of coal in the furnacev and assuming that the heat transfer agentfiowing through the tubes K has the Specific-heat of a high flashing point kerosene, the-maximum heat transfercapacity'of the apparatus will be obtained when the velocity of flow of the transfer agent through the minute.

for use with tubesthree sixteenths of an inch in external diameter, and with tube walls one thirtysecond of an inch thick, so that the diameter of the tube tubes Kis about 530 feetv per the specific heats of the heating gases and of -Those skilled in the art will understand from the foregoing explanation, as well as from the dimensions stated above by way of example, that the use of the invention makes it possible to obtain a given heat transfer effect in air heaters and analogous apparatus the bulk of which is. but a small fraction of the bulk of apparatus of the character now in use for obtaining the same heat transfer effect. 1

Those skilled in the art will understand that with the small diameter and close spacing of the tubes K in the device 1 that device would quickly become inoperative from a practical standpoint, as a result of the ac-v cumulation of furnace dust in the device unless provisions were made for frequent and adequate cleaning. The best and most effective method of cleaning such apparatus is by 'washing it down with water, and for this purpose I have provided spray pipes O for discharging cleaning'water above the tubes.

Advantageously these spray pipes are arranged to permit sectional cleaning of the device F. For example, as shown in Fig. fl, there are two spray pipes 0 above each un t 7, and each spray pipe is individually connected to a supply pipe or header 0' by an individual cut-off valve 0 With this arrangement when any one unit 7 is-..being cleaned the resultant disturbances in draft and other conditions of operation are confined to the unit being cleaned, and the 'remaining' units may continue to operate in the 7 normal manner. While the tendency of dust to accumulate on the tubes K ismuch less marked in the device FA than in the tubes F, I consider it ordinarily desirable to supply the tubes FA with water spray cleaning pipes similar to those used with the device F.

The tubes K may be formed of aluminum in a satisfactory and relatively inexpensive manner. and'when the tubes K of either device F or FA are formed of aluminum it is desirable to make the rest of the deviceof the same metal to avoid electrolytic action. The relatively small bulk of the apparatus and the conditions of use are such as to make alumi-- num a relatively inexpensive and desirable metalout'of which'to make those devices;

' particularly as aluminum is not subj ect to corthe statutes I have illustrated and described herein the best embodiments of my invention now known tome, it will be'apparent'to those skilled in theart that many formal changes may be made without departing from the spirit of my invention as set forth. in the 'appended claims, and'that certain features-of my invention maysometimes be used with advantage without a corresponding use of other features. I amaware of proposals furnace heating gases to dium for transferring heat from one fluid.

to another, butl'I do not regard sand as practically feasible for such use because sand does not flow and cannot be circulated as. a liquid or a gas flows and can be circulated, and I intend the term fluid, as used in this specifi cation and in the appended claims to include liquids and gases but to exclude sand and analogous solid substances.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. The method of effecting a heat transfer between two fluids at different temperatures which consists in passing a fluid heat transfer medium along a path of flow including a portion in which said medium absorbs heat from the hotter of the two first mentioned fluids and a portion in which said medium imparts heat to the cooler of the two first mentioned fluids, and dividing the fluid heat transfer medium into a multiplicity of fine streams of predetermined fineness in each of said port1ons, A r

2. The method of transfer ing heat from a hotter fluid to a cooler fluid'which consists in passing a fluid heat transfer-medium along a path of flow comprisinga portion in which said medium absorbs heat from the hotter fluid and a second portion in' which the medium gives up heat to the cooler fluid, and in each of said portions dividing said medium into streams which are fine enough so that "nace waste heating gases to preheat the air supplied the furnace for combustion therein, which consists in transferring heat from said gases to a li uid heat transfer medium without apprecia le vaporization of said medium and then transferring heat from said medium to the air to be preheated.

4. The combination with the heating gas outlet and air intake flues of a-fnrnace, of heat transfer apparatus. comprising a circulating system including a heat absorbing section in said outlet flue and a cooling section in said intake fine and means for circulating a fluid through said system.

5. Apparatus for utilizing the heat in waste preheat the air supphed the furnace for com ustion, comprising a heat absorbing portion traversed by' the waste gases and an air heatin portion trav- .ersed by the air supplied the urnace to-heat said combustion a1r, each of said portions comprising a multlphcit of small tubes, and

nace waste heating gases to preheat the air supplied for combustion therein which consists in passing a liquid heat transfer medium into heat absorbing counter current flow relation with the heating gases and then into heat dispensing counter current flow relation with the air to be preheated, with a weight rate of fiow which is in approximately the same proportion to the weight rate of flow of' the heating gases as exists between the specific heats of the heating gases and transfer agent.

7. The method of operating heat transfer apparatus comprising one heat exchanger operating on the counter current principle for transferring heat from one fluid medium to a second fluid medium, and a second heat exchanger operating on the counter current principle through which the second medium is passed and from which it is returned to the first heat exchanger and in which the second medium transfers heat to a third medium, the improvement which consists in making the weight rate of flow of the second medium apof the latter; C represents the product of the 31 heat transfer rate in the second heat exchanger and the heat dispensing surface area of the latter; D represents the product of the weight rate of flow and specific heat of the first medium, and E represents the'product of the weight rate of flow and the specific heat of the third medium.

Signed at New York City, in the county of New York and State of New York this 14th day of September, A. D. 1923.

JOHN E. BELL. 

